Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical printing plate precursor for computer-to-film methods comprise a hydrophilic support and an image-recording layer of a photosensitive polymer which include UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’. Also positive-working materials, wherein the exposed areas define the non-printing areas, are known, e.g. plates having a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed areas.
In addition to the above photosensitive materials, also heat-sensitive printing plate precursors have become very popular. Such thermal materials offer the advantage of daylight-stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilisation by cross-linking of a polymer, heat-induced solubilisation, decomposition, or particle coagulation of a thermoplastic polymer latex.
The known heat-sensitive printing plate precursors typically comprise a hydrophilic support and a coating containing an oleophilic polymer, which is alkali-soluble in exposed areas (positive working material) or in the non-exposed areas (negative working material) and an IR-absorbing compound. Such an oleophilic polymer is typically a phenolic resin.
WO 97/39894 describes a positive-working heat-sensitive printing plate precursor which is sensitive to IR light but not to UV light comprising a support and an IR-sensitive coating comprising an oleophilic polymer that is soluble in an aqueous alkaline developer and a dissolution inhibitor which reduces the solubility of the polymer in the developer.
EP-A 0 864 420 describes a positive-working heat-sensitive printing plate precursor comprising a support, a first layer containing an oleophilic polymer that is soluble in an aqueous alkaline developer and an IR-sensitive top layer of which the penetrability by or solubility in the aqueous alkaline developer is changed upon exposure to IR light.
EP-A 0 934 822 describes a photosensitive composition for a lithographic printing plate wherein the composition contains an alkali-soluble resin having phenolic hydroxyl groups and of which at least some of the phenolic hydroxyl groups are esterified by a sulphonic acid or a carboxylic acid compound.
EP-A 1 072 432 describes an image forming material which comprises a recording layer which is formed of a composition whose solubility in water or in an alkali aqueous solution is altered by the effects of light or heat. This recording layer comprises a polymer of vinyl phenol or a phenolic polymer, wherein hydroxy groups and alkoxy groups are directly linked to the aromatic hydrocarbon ring. The alkoxy group is composed of 20 or less carbon atoms.
U.S. Pat. No. 5,641,608 describes a direct process for producing an imaged pattern on a substrate surface for printed circuit board application. The process utilises a thermo-resist composition which undergo a thermally-induced chemical transformation effective either to ablate the composition or to increase or decrease its solubility in a particular developer. The thermo-resist composition comprises phenolic polymers in which free hydroxyl groups are protected. Upon heating in the presence of an acid these protecting groups split off resulting in a solubility change of the composition. In positive thermo-resists the hydroxyl protecting groups may be ethers, such as alkyl-, benzyl-, cycloalkyl- or trialkylsilyl-ethers, and oxy-carbonyl groups.
EP-A 0 982 123 describes a photosensitive resin composition or recording material wherein the binder is a phenolic polymer, substituted with a specific functional group on the aromatic hydrocarbon ring such as a halogen atom, an alkyl group having 12 or less carbon atoms, an alkoxy group, an alkylthio group, a cyano group, a nitro group or a trifluoromethyl group, or wherein the hydrogen atom of the hydroxy group of the phenolic polymer is substituted with a specific functional group such as an amide, a thioamide or a sulphonamide group. As a result, the coating of the recording material has such a high density that improves the intra-film transistivity of heat obtained by the light-to-heat conversion at the time of laser exposure. The high density of the coating makes the image recording material less susceptible to external influences such as humidity and temperature. Consequently, the storage stability of the image recording material can also be enhanced.
U.S. Pat. No. 4,939,229 describes a method for the preparation of branched novolacs, useful for photoresist compositions, by reacting a tris- or tetrakis(dialkylaminoalkyl)phenol with a phenolic compound in the presence of an acid catalyst. Due to the reaction with these intermediate dialkylaminoalkyl-phenol compounds, a reproducible method for the synthesis of branched novolacs is obtained.
WO99/01795 describes a method for preparing a positive working resist pattern on a substrate wherein the coating composition comprises a polymeric substance having functional groups such that the functionalised polymeric substance has the property that it is developer insoluble prior to delivery of radiation and developer soluble thereafter. Suitable functional groups are known to favor hydrogen bonding and may comprise amino, amido, chloro, fluoro, carbonyl, sulphinyl and sulphonyl groups and these groups are bonded to the polymeric substance by an esterification reaction with the phenolic hydroxy group to form a resin ester.
EP-A 02 102 446, filed on 15 Oct. 2002, EP-A 02 102 444, filed on 15 Oct. 2002, EP-A 02 102 445, filed on 15 Oct. 2002, EP-A 02 102 443, filed on 15 Oct. 2002 and EP-A 03 102 522, filed on 13 Aug. 2003, describe positive-working heat-sensitive lithographic printing plate precursors wherein the coating comprises phenolic resins which are modified by various substituents that improve the chemical resistance of the coating, i.e. which render the coating less vulnerable to attack by the organic chemicals that are typically present in fountain solutions, plate cleaners, blanket wash liquids, etc. Such substitution however typically produces a reduction of the sensitivity of the plate because also the resistance of the coating towards the developer is increased: in positive-working lithographic printing plate precursors the exposure energy required for rendering the exposed areas of the coating soluble in the developer determines the sensitivity of the precursor. In order to compensate for this sensitivity decrease by said substitution, it is known to add development accelerators such as cyclic acid anhydrides, phenols or organic acids. These low molecular weight compounds increase the rate of dissolution of the exposed areas, but this measure on its turn also reduces the developer resistance of the unexposed areas. A high developer resistance of the unexposed areas is advantageous because it results in a high developer dissolution contrast, also called development latitude: advantageously, the exposed areas of the coating are completely dissolved in the developer before the non-exposed areas are affected by the developer.
In summary, it remains a problem to provide a thermal positive-working lithographic printing plate precursor with both a high sensitivity and a high developer dissolution contrast.